It is tricky to create an infinity symbol (or lemniscate) in PowerPoint, it is a shape that needs to overlap with itself and requires Escher-style (impossible) layering of shapes. The only way to do it is cheat, and construct the final shape of many individual shapes that are grouped together cleverly.
I managed to get it done, and you can see the final result here (hmm, those arrows point the wrong way around though):
An infinity symbol in PowerPoint
I don't have the exact workflow anymore that I used (I made some destructive edits), but below is a screenshot of the PowerPoint file in slide sorter mode that I used to create the shape, starting with 2 circles and a square.
How to create an infinity shape in PowerPoint
This shape is useful to show concepts that keep on going, or loops that you can't get out of. You can download the infinity symbol here, or find other slides with loops. There are Apple Keynote versions available as well.
Cover image by Mark Asthoff on Unsplash